Extremely thin, patterned or irregular shaped metallic foils are difficult to fabricate by conventional manufacturing techniques. Patterns can be pressed into metal foil using a metal embossing cylinder, but such methods are often not suitable for extremely fine detailed patterns.
Plating thin metal coatings on aluminum substrates is one method of making thin wall shapes. Most plating processes, such as that taught by Coll-Palagos in U.S. Pat. No. 3,726,771, require removal of the oxide on the aluminum, for example by a HF dip, and then metal deposition under conditions that prevent reoxidation of the aluminum substrate surface. In such a process, the deoxidizing can easily destroy the original finish and detailed pattern or grain of the metal substrate. The pattern on the electroformed foil will be fuzzy and lack fine detail. Cleaning processes, such as a simple room temperature nitric acid dip followed by application of a zinc petrolatum compound, as taught by Bonwitt in U.S. Pat. No. 2,437,220, while cleaning without removing all the oxide, do not provide a particularly suitable electro-deposition surface.
Cooke et al, in U.S. Pat. No. 3,718,547, teaches a continuous process for selectively removing magnesium oxide from a magnesium containing aluminum substrate, and then reforming an anodic oxide film by alternating current electrical treatment in sulfuric acid at 90.degree.C, prior to lacquering. Such initial oxide removal could easily destroy the original finish at many points on the surface.
Bailey et al, in U.S. Pat. No. 3,844,906, rather than remove the aluminum oxide layer from a cylindrical mandrel, plates chromium over it and polishes the chromium, prior to electroforming a nickel coating from a nickel sulfamate bath at 200 to 500 A/sq. ft. This process generates high bath temperatures requiring cooling, and involves the expense of chromium plating and polishing, which plating would not conform to any fine detail present on the mandrel.
What is needed then, is a method of manufacturing thin metal foil which can successfully reproduce an extremely fine detailed pattern or mirror finish on the foil, from a specially treated patterned, embossing plate or mandrel having a surface conducive to metal electro-coating.